Obesity results from an imbalance between energy intake and energy expenditure, and both of the factors have been studied. the major factor that can alter the energy expenditure is physical activity or exercise, therefore this factor could be important in understanding obesity development and maintenance. Despite this realization, very little is known about the relationship between daily physical activity and body weight regulation. Although epidemiological research certainly suggests there could be such a relationship, previous experimental research has not successfully defined its nature. Major limitations have been the inability to accurately quantify the amount of daily physical activity performed by human subjects, and the failure to accurately measure the energy expenditure associated with these activities. To overcome these limitations, we developed a new method for quantifying the amount of physical activity (work) performed by human subjects living inside a whole-room indirect calorimeter. A special force platform has been built and validated inside the room calorimeter which measures the work performed by human subjects. A computer-controlled system has been developed to analyze the force, work and other activities of the subject inside the calorimeter. In addition, the response time of the whole-room indirect calorimeter has been vastly improved to provide accurate measurements of energy expenditure over 1-minute intervals. The new system is now able to: 1) simultaneously and accurately measure the dynamic changes in work and energy expenditure inside the room calorimeter on 24-hr basis; 2) calculate work efficiency as work performed divided by the increase in related energy expenditure; 3) quantify the amount of energy expended in physical activity, which is determined by the amount and efficiency of work. The intent of this project is to use our new and improved systems to study the role of physical activity in body weight regulation. In the first 3 years of this project, we plan to study a wide range of subjects in our calorimeter to determine reasons for within- and between-subject variation in amount and efficiency of work. Only with this information can we design intervention studies to further investigate the role of physical activity in obesity. In years 4-5, we plan to use the information generated in the first 3 years to begin to understand how important the energy expended in physical activity is in body weight regulation, and whether this energy component can be altered by altering its determinants obtained in years 1-3. According to our preliminary studies, we have proposed to examine how the energy expended in physical activity changes with weight loss and with exercise training. Interventions on other determinants will also be conducted if they are identified in years 1-3. Throughout this project, the effect of exercise on the changes in total energy expenditure and its components will also be studied. We believe the new work-energy measurement system will allow us to make significant progress in understanding the relationship between physical activity and obesity.